Bump flake8 from 3.9.2 to 4.0.1 (#631)

Bumps [flake8](https://github.com/pycqa/flake8) from 3.9.2 to 4.0.1.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](PyCQA/flake8@3.9.2...4.0.1)

---
updated-dependencies:
- dependency-name: flake8
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/test.txt

@@ -2,7 +2,7 @@ async-timeout==3.0.1
 pytest==6.2.4
 pytest-cov==3.0.0
 pytest-asyncio==0.16.0
-flake8==3.9.2
+flake8==4.0.1
 mypy==0.910
 black==21.9b0
 isort==5.9.3